Measurements requiere the neutrinos to be relativistic at recombination, making the evolutional history practically indistinguishable for
\(\sum m_\nu \le 1.8\,\mathrm {eV}\)  before recombination.
However, after recombination the evolution strongly depends on the absolute mass scale as this directly translates into a transition time between the relativistic and non-relativistic propagation.
Non-relativistic neutrinos, since they behave as a matter component of our Universe, contribute to the hot dark matter content and are severely constraint by the baryonic acoustic oscillations.
Furthermore, the current cosmological bound is robust against common cosmological extension of the \(\Lambda CDM\)  model, as e.g. the inclusion of dark radiation or non-standard dark energy dynamics.
Nevertheless, a modification of the particle physics properties of the neutrino, as e.g. cosmological fast decays, can dilute the late time neutrino energy density and hence evade the main constraining power, which is represented by the dashed lines [1]}.
